Dave Filoni had a massive undertaking with "The Clone Wars." He needed to tell a compelling story that perfectly led up to the events of "Revenge of the Sith" fans knew so well. The solution was for Ahsoka Tano to leave the Jedi Order before that even happened. In "The Clone Wars" Season 5, Ahsoka feels betrayed by the Jedi Council after she's framed for a bombing. She is innocent, but the damage is done, and she leaves Anakin, pursuing her own path.
With this in mind, it makes some sense why no one in the Jedi Order present in "Revenge of the Sith" references her. She's long gone, so they all have other things to worry about at this tumultuous time. However, Ahsoka was definitely still alive, and "The Clone Wars" shows what she was up to when Order 66 occurred. She was helping liberate Mandalore from Darth Maul, eventually defeating him and installing Bo-Katan Kryze as leader. She's on a ship going to Coruscant when Order 66 is activated. The Stormtroopers turn on her, and she has to fight to survive, freeing Darth Maul in the process.
While Anakin and Obi-Wan Kenobi (Ewan McGregor) dueled on Mustarfar, Ahsoka had her own problems. But she was still very much a hero during the events of the original trilogy. "Star Wars Rebels" even shows how she battled her former master, now known as Darth Vader. Ahsoka has been present for numerous important eras of the "Star Wars" story, and with any luck, we may see what she was up to during the sequel trilogy.
